(a) Technical Field
The present disclosure relates, generally, to a fuel cell system. More particularly, it relates to a method for determining the occurrence of ice blocking in real time, wherein an electrode surface of a fuel cell stack is frozen out to cut off the supply of reactant gases.
(b) Background Art
A typical fuel cell system applied to a hydrogen fuel cell vehicle preferably comprises a fuel cell stack for generating electrical energy by an electrochemical reaction of reactant gases, a hydrogen supply system for suitably supplying hydrogen as a fuel to the fuel cell stack, an air supply system for suitably supplying air containing oxygen as an oxidant required for the electrochemical reaction in the fuel cell stack, a thermal management system (TMS) for suitably removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management function, and a system controller for suitably controlling overall operation of the fuel cell system.
Preferably, the hydrogen supply system includes a hydrogen tank, high-pressure and low-pressure regulators, a hydrogen valve, a hydrogen recirculation system, etc., the air supply system includes an air blower, an air valve, a humidifier, etc., and the TMS includes a coolant pump, a radiator, etc.
Preferably, in the hydrogen supply system, high pressure hydrogen supplied from the hydrogen tank sequentially passes through the high-pressure and low-pressure regulators and then is suitably supplied to the fuel cell stack at a low pressure. Preferably, in the hydrogen recirculation system, a blower provided in a recirculation line recirculates unreacted hydrogen of an anode (“fuel electrode” or “hydrogen electrode”) of the fuel cell stack to the anode, thus recycling the hydrogen.
In the air supply system, dry air supplied by the air blower passes through the humidifier to be humidified by absorbing water from exhaust gas discharged from a cathode (“air electrode” or “oxygen electrode”) of the fuel cell stack and then is supplied to the cathode of the fuel cell stack.
An urgent and serious consideration of the fuel cell vehicle having the above-described fuel cell system is to ensure cold startability.
In particular, when an electrode surface of a membrane electrode assembly (MEA) is frozen out in a certain cell due to water remaining in the fuel cell stack during cold start of the fuel cell vehicle, a flow field of the corresponding cell in the fuel cell stack is shut off by the frozen ice, thus cutting off the supply of reactant gases (hydrogen and air).
For example, when a load is applied to the fuel cell system, which has been exposed to sub-zero temperatures for a long time, during initial start-up, water produced in the cathode by the electrochemical reaction is frozen by cold air of the fuel cell stack itself and air at a temperature below the freezing point supplied to the cathode, thus blocking various flow fields and a gas diffusion layer of the fuel cell stack and, at the same time, cutting off the air supply to a catalyst layer of the cathode. As a result, the voltage of the fuel cell stack is not suitably maintained constant, which makes it difficult to ensure the cold startability.
If an electrical load is suitably applied to the fuel cell stack during cold start under the conditions where the supply of reactant gases is cut off due to the frozen electrode surface, i.e., in the event of ice blocking, a reverse voltage is applied to the fuel cell stack, which damages the fuel cell stack, and as a result this can have an adverse effect on the durability.
However, in certain examples where ice blocking occurs in the cathode due to the frozen electrode surface, only the air supply is cut off, and thus it is possible to obtain considerable output power only with the reaction of hydrogen without deterioration of the fuel cell stack. Accordingly, the reverse voltage in this case can preferably be ignored. Further, since it is possible to use the output power of the fuel cell stack without suitable deterioration of the catalyst layer, it is possible to suitably perform limited operations such as the cold start process and the vehicle running.
However, in the case that the ice blocking occurs in the anode electrode, the hydrogen supply is cut off, and thus the catalyst layer of the electrode surface is suitably damaged even with a relatively small load. Accordingly, it is necessary to perform an emergency shut-down process immediately.
Accordingly, a technique for distinguishing the ice blocking in the anode (hereinafter referred to as “anode ice blocking”) from the ice blocking occurring in the cathode (hereinafter referred to as “cathode ice blocking”) is needed in the art.
FIG. 2 is a graph showing the voltages of cells in which the ice blocking occurs due to the frozen electrode surface. It is possible to distinguish the anode ice blocking from the cathode ice blocking by comparing the reverse voltages generated in each single cell.
Preferably, when examining the reverse voltage behavior due to a lack of air on the electrode surface of the cathode during the cathode ice blocking, it can be seen that although the reverse voltage is generated, a constant voltage is suitably maintained near −0 V for several seconds or minutes and the cathode electrode may not be deteriorated by this reverse voltage behavior.
However, a sudden reverse voltage (above −1 V) is generated during the anode ice blocking compared to the lack of air in the cathode, and thereby the catalyst layer of the electrode may be deteriorated.
Preferably, in order to distinguish the anode ice blocking from the cathode ice blocking, it is necessary to measure the reverse voltage of each single cell in the fuel cell stack in which a plurality of unit cells are stacked.
In particular, it is difficult to install a complicated and expensive voltage detector in each cell to measure the reverse voltage of each single cell. Even though it is installed in each single cell, the configuration of the fuel cell system applied to a vehicle is complicated, and the cost of the vehicle is considerably increased.
Accordingly, it is difficult to distinguish the anode ice blocking from the cathode ice blocking by measuring the reverse voltage of each signal cell in the fuel cell stack and control the vehicle using the same.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.